MIMO-OFDM communication schemes include a clustered OFDM scheme, a space-time trellis code OFDM scheme (STTC-OFDM), space-time block code OFDM scheme (STBC-OFDM), space-frequency block code OFDM scheme (SFBC-OFDM), etc.
The MIMO-OFDM has advantages in increasing a link budget, simplifying a receiving terminal, decreasing a peak-to-average power ratio (PAPR) of a transmitting terminal, etc. Here, the simplification of the receiving terminal does not mean a simplification of hardware, but rather a simplification of a mathematical process. When embodying hardware, there must be as many FFT operators in the receiving terminal as there are antennae. Accordingly, the size of a transmitting/receiving terminal becomes large.
FIG. 1 shows a schematic diagram of a structure of the MIMO-OFDM system.
The MIMO-OFDM system includes a transmitting terminal and a receiving terminal. The transmitting terminal includes a MIMO-OFDM encoder 20, n number of IFFT operators 30-1, 30-2, . . . , and 30-n, and n number of transmitting antennae 40-1, 40-2, . . . , and 40-n. The receiving antennae includes n number of receiving antennae 50-1, 50-2, . . . , and 50-n, n number of FFT operators 60-1, 60-2, . . . , and 60-n, and a MIMO-OFDM decoder 70.
The MIMO-OFDM encoder 20 receives a transmitting information bit array 10, and generates a transmitting data symbol.
The IFFT operators 30-1, 30-2, . . . , and 30-n receive the transmitting data symbol, and generate a transmitting OFDM symbol.
The FFT operators 60-1, 60-2, . . . , and 60-n receive a receiving OFDM symbol from the receiving antennae 50-1, 50-2, . . . , and 50-n, and generate a receiving data symbol.
The MIMO-OFDM decoder 70 receives the receiving data symbol, and generates a receiving information bit array 80.
In an OFDM modem, in order to generate an OFDM symbol which is regarded as an OFDM signal, a symbol of which an original bit array is QAM-modulated (or QPSK-modulation, 16-QAM-modulation, 64-QAM-modulation, and etc.) is used. Hereinafter, the symbol will be referred to as a transmitting data symbol. In addition, the IFFT operator receives the transmitting data symbol, and generates an OFDM symbol. Here, the OFDM symbol will be referred to as a transmitting OFDM symbol.
Similarly, an OFDM symbol received from the receiving antenna will be referred to as a receiving OFDM symbol, and a QAM symbol which is generated by the FFT operator with the receiving OFDM symbol will be referred to as a receiving data symbol.
As shown in FIG. 1, a terminal needs n number of FIT operators for receiving. Since the FFT operator and the IFFT operator need a considerable amount of logic, an increase of the number of the operators causes a large load on hardware, especially on terminals.
An easy method for reducing the number of FFT operators or IFFT operators is to increase an operation speed of the operator. When the FFT operator is operated at a 4 times higher speed, the number of FFT operators may be reduced to ¼. However, there is a limit to the operation speed of the FFT operator, and a higher speed FFT operator generally has a more complex structure. Therefore, the operation speed of the FFT operator cannot be increased without limit.
FIG. 2 is a block diagram showing that the IFFT operator and the FFT operator can be provided in a single apparatus.
As shown in FIG. 2, when respectively setting a real number input and an imaginary number input of the FIT operator 110 to be an imaginary number input and a real number input of the IFFT operator 100, and respectively setting a real number output and an imaginary number output of the IFFT operator 110 to be an imaginary number output and a real number output of the IFFT operator 100, the IFFT operator 100 can be realized by the FFT operator 110. Also, when arranging contrarily, it is clear that the FIT operator can be realized by the IFFT operator.
Hereinafter, the apparatus designed to perform as both the FFT operator and the IFFT operator will be referred to as an FFT/IFFT operator. As shown in FIG. 2, in order to embody one FFT/IFFT operator, only logic for exchanging input/output is additionally necessary in addition to hardware logic for one FFT operator 1.
When using the FIT/IFFT operator, a receiver and a transmitter in a time division duplex (TDD) system may co-own the same FFT/IFFT operator.
FIG. 3 is a symbol layout view when respectively using the IFFT operator and the FFT operator for the transmitter and the receiver in the TDD system.
A transmitting/receiving symbol stream shows a time domain in which a radio channel is occupied by receiving OFDM symbols 1, 2, 3, 4, and 5, and transmitting OFDM symbols 6, 7, and 8. The receiving OFDM symbols 1, 2, 3, 4, and 5 are allocated to time slots 1, 2, 3, 4, and 5 of a FFT schedule, and are demodulated. The transmitting OFDM symbols 6, 7, and 8 are generated in time slots 6, 7, and 8 of an IFFT schedule through a modulation process.
As shown in FIG. 3, in the TDD system, due to a time for preparing the transmitting OFDM symbols 6, 7, and 8, durations in which rear part symbols 4 and 5 of the receiving OFDM symbols use the FFT operator and durations in which front part symbols 6 and 7 of the transmitting OFDM symbols use the IFFT operator are overlapped. Therefore, in a typical OFDM system, a time-sharing design for operation of the FFT/IFFT operator is necessary in order to process a transmitting/receiving process by using the FFT/IFFT operator.
FIG. 4 is a symbol layout view showing that the transmitter and the receiver co-own one FFT/IFFT operator when the operation speed of the FFT/IFFT operator is the same as a data transmission speed.
In order that the transmitter and the receiver may co-own one FFT/IFFT operator without time-sharing for the operation of the FFT/IFFT operator, the rear part symbols 4 and 5 of the receiving OFDM symbol need to be scheduled to process FFT operations in time slot 4 and 5 after the transmitting OFDM symbols 6, 7, and 8 are generated through the FFT/IFFT operator, as shown in FIG. 4. However, in this case, a problem occurs that processes for later demodulated symbols are delayed. After FFT operation, channel estimation, equalization, QAM demapping, channel decoder operation, and MAC layer operation also take time. Here, when some symbols are demodulated later, the whole demodulation process may be delayed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.